1. Field of the Invention
The present invention relates to a system for measuring electromagnetic radiation originating from the hemisphere (corresponding to a solid angle of 2.pi.).
2. Description of the Prior Art
In the meteorological field, among others, measurement of solar and terrestrial radiation is conducted over wave lengths ranging between approximately 0.3 .mu.m to 3 .mu.m, and from 3 .mu.m to 100 .mu.m, respectively, so as to determine the radiation balance, i.e. the difference between radiation incident on the earth's surface and radiation emitted or reflected thereby, as well as the radiation components. For this purpose, any radiation from the upper and lower hemisphere (each corresponding to a solid angle of 2.pi.) must be detected and measured. Presently, such measuring operations are conducted not only by stationary means but, to an ever increasing degree, also with the aid of aircraft or spacecraft.
Heretofore, many different types of devices have been proposed for such measuring operations. As diagrammatically shown in FIG. 1, such a device may comprise for example a thermopile T, the horizontally disposed active soldered joints of which are blackened and insulated against the remaining component parts of the device. On incidence of radiation, the temperature of the active soldered joints rises above the temperature of the passive soldered joints, the latter being in thermal connection to a relatively large mass of the device. The resulting thermoelectric voltage, optionally together with other measured variables such as the temperature of passive soldered joints, serves as a measure of incident radiation. To prevent heat exchange with the surrounding atmosphere either by advection or convection, thermopile T is usually covered by a protective dome made of a material that is transmissive of the radiation to be measured, such as glass, polyethylene or the like. To secure even better protection against heat exchange, thermopile T may be covered by two hemispherical domes K1 and K2 according to a different conventional embodiment, both domes being again made of a transmissive material.
Devices of the above type operating by continuous light detection not only show relatively high inertia which is noticeable e.g. on use in aircraft; it is a particular disadvantage of such devices that variations in the temperature of the device itself and especially in the temperature of the not completely transparent domes may have a strong effect on the measuring signal. It was to minimize this effect that installation of the two concentric domes K1 and K2 shown in FIG. 2 was proposed.
The disadvantages of the prior art devices caused by their inertia and the effect of temperature on casing and dome might be overcome by employing the so-called chopped-light detection method. This method is state of the art but hitherto it could not be used for measuring radiation from the hemisphere (corresponding to a solid angle of 2.pi.), being suited only for devices having a limited aperture angle.
A system employing the conventional chopped-light detection method is shown in FIG. 3 and includes a radiation detector D of high sensitivity and very low inertia, e.g. a semiconductor bolometer, photoelectric cell, or pyroelectric detector, mounted behind an oscillating or rotating chopper MS which alternately covers and exposes detector D to radiation. According to the embodiment of FIG. 3, radiation is concentrated by means of a lens system 0 positioned in front of detector D. In this conventional embodiment, there is generated at detector D either alternating voltage or alternating current corresponding to the difference between radiation measured and radiation emitted and/or reflected by modulator disk MS (the latter being the so-called comparative radiation). It is an advantage of the above method that emission changes within the system (i.e. at detector, casing, window or optical system), due for example to temperature variations, have the same effect on measured radiation and comparative radiation and, thus, will not be impressed on either alternating voltage or alternating current. Some disadvantage may still be seen in the fact that the radiation emitted and/or reflected by the modulator disk must be known as it constitutes the reference signal (zero signal).